This invention pertains generally to fusion devices and methods and is more particularly directed to such devices and methods wherein high energy beams of positive hot ions are injected into a confining magnetic field wherein the ion beams are trapped in orbits where the ions react to release energy in spontaneous fusion reactions.
There are many fusion devices based on various confinement configurations and confinement principles wherein plasma is generated in a reaction chamber and confined magnetically. The plasma is heated by such methods as ohmic heating, r.f. heating and neutral beam heating to temperatures where the nuclei in the plasma react to release energy. Deuterium and tritium nuclei, that is deuterons and tritons, are the common reactants, for upon fusion they produce an alpha particle and a neutron and more than 17 MeV in energy, about 14 MeV in the form of the kinetic energy of the neutron and the rest in the form of the kinetic energy of the alpha particle. This energy is commonly captured in a blanket and converted to heat used to generate useful electricity. A major problem with such devices lies in confining the plasma long enough for enough reactions to occur to justify the energy needed to operate the devices, of which operation of the confining magnetic fields is a large part. Among such devices are those of toroidal geometry, such as tokamaks, and those of linear geometry, such as mirror machines.
In tokamaks the plasma is confined in a toroidal chamber primarily by the combination of toroidal and poloidal fields. In general, the toroidal field directs the plasma around the torus, and the poloidal field keeps the plasma together. The plasma ions follow the magnetic field lines around the torus, orbiting the respective toroidal field lines at radii small compared to the dimensions of the chamber. The magnetic field lines have a rotational transform ##EQU1## Where r is the minor radius of the plasma, R is the major radius; B.sub.t is the toroidal magnetic field and B.sub.p is the poloidal magnetic field. The Kruskal-Shafranov condition is that .gamma.&lt;2.pi.. When .gamma.=2.pi., there is no rotational transform of the magnetic field to compensate the toroidal drift of the particles or ions.
Alternatively, tokamak instabilities are associated with k.B=(n/R)B.sub.t .+-.(m/r)B.sub.p =0, where n and m are integers. This condition means that particles or ions following the field lines remain in resonance with the perturbation characterized by the integers m and n. The quantity q=2.pi./.gamma. is known as the safety factor, and an essential restriction for toroidal confinement is q&gt;1. This means that the toroidal field ##EQU2## must be very large. For example, with a safety factor q=4, the magnetic pressure B.sub.p.sup.2 /8.pi. equal to the particle pressure nT (that is, B.sub.p.sup.2 /8.pi.=nT), ion density n=2.times.10.sup.14 cm .sup.-3, and ion temperature T=50 keV, B.sub.p =20 kG. To keep the toroidal field B.sub.t below 100 kG, R/r must be near unity. The inescapable result of this is that in tokamaks an enormous energy investment in the toroidal field energy must be made which serves no useful purpose except that it is necessary for stability. The requirement that q&gt;1 is the principal reason for poor economics of the tokamak geometry fusion reactor. Therefore, it would be extremely advantageous to provide a magnetic containment configuration which does not require the extremely large toroidal field of the tokamak.
There have been devices for producing fusion reactions by bombarding a target plasma with energetic ion beams. This is usually in the context of a dense background plasma with hot electrons and cold ions. Because of the cold plasma ions, the beam ions slow down rapidly, and the maximum gain of fusion energy/beam ion energy for the device is limited to 3-4. Such a device is shown in Dawson, J. M., H. P. Furth and F. H. Tenney, "Stellarator-Mirror Machine Target Plasma Reactor," Plasma Physics Laboratory, Princeton University, Princeton, N.J., MATT-841, May 1971.
Another device wherein an ion beam is directed against a plasma is shown in Manheimer et al., U.S. Pat. No. 4,548,782, wherein an ion accelerator produces a neutral beam used to heat a magnetically confined plasma in a tokamak. In one disclosed embodiment the plasma is at low density when the beam is injected, and the remaining plasma is built up around it thereafter. Confinement is according to tokamak principles, and the plasma is heated to ignition.